1. Field of the Invention
The present invention relates to gas sensor elements capable of detecting a concentration of a specific gas component contained in a target detection gas such as exhaust gas emitted from an internal combustion engine, and relates to gas sensors equipped with the gas sensor element.
2. Description of the Related Art
For example, a conventional gas sensor is equipped with a gas sensor element. For example, Japanese patent laid open publication No. JP 2006-38496 has disclosed such a conventional gas sensor. The gas sensor element in the gas sensor is comprised of a solid electrolyte body, a detection electrode, a reference electrode, an electrode protection porous layer, and an anti-poisoning porous layer. The solid electrolyte body has oxygen ion conductivity. The detection electrode is formed on one surface of the solid electrolyte body and is in contact with a target detection gas. The reference electrode is formed on the other surface of the solid electrolyte body and is in contact with a reference gas. The detection electrode is covered with the electrode protection porous layer. The electrode protection porous layer supports catalyst metal particles. The electrode protection porous layer is covered with the anti-poisoning porous layer. The anti-poisoning porous layer protects the catalyst metal particles from poisoning.
The conventional gas sensor having the above structure can be used as a gas sensor which detects a concentration of oxygen or nitrogen oxide NOx contained in exhaust gas emitted from an internal combustion engine of a motor vehicle. During the beginning of engine start, the exhaust gas contains a large quantity of hydrogen. In general, hydrogen moves at high speed through the electrode protection layer in the gas sensor element compared to other gas components such as oxygen contained in the target detection gas. Accordingly, because such hydrogen reaches and reacts to the detection electrode before other gas components reach the detection electrode, the gas sensor often causes error detection, and detects an incorrect λ-point which is shifted from a true λ point. That is, the conventional gas sensor has a possibility of causing a λ-shift.
In order to avoid such a λ-shift problem, the conventional gas sensor, for example, disclosed in Japanese patent laid open publication No. JP2006-38496, catalyst metal particles are supported on the electrode protection layer. This structure of the conventional gas sensor element forcedly places hydrogen contained in the target detection gas in reaction with the catalyst metal particles supported on the electrode protection layer, and this structure prevents hydrogen from reaching the detection electrode.
By the way, there is a cup-shaped gas sensor having the above characteristics is comprised of a leg part, as the solid electrolyte body, having a cylindrical shape extending along its axial direction, and a bottom part (or a closed base) with which the front of the leg part is closed. In such a cup-shaped gas sensor, a longitudinal shaped heater is inserted and placed in the inside of the solid electrolyte body. When receiving electric power, the longitudinal shaped heater generates heat energy in the gas sensor element, and the heat energy activates oxygen ion conductivity and catalyst layer reaction characteristics of the solid electrolyte body.
During the beginning of the engine start, exhaust gas as the target detection gas is hydrogen rich, and the bottom part at the front side of the gas sensor element has a high temperature rising speed when compared with the leg part. As a result, the gas sensor element has a temperature change distribution, and does not have a uniform temperature distribution.
Accordingly, during the beginning of the engine start, because the catalyst metal particle placed at the bottom part is rapidly activated, it is difficult to adequately suppress hydrogen from reaching the detection electrode. It is thereby difficult to avoid such a λ-shift problem.
In addition, because gas adsorbed in the catalyst is also increased and on the other hand the diffusion speed of the target detection gas is decreased when the quantity of catalyst supported on the catalyst layer in the electrode protection layer is increased, the response characteristics of the gas sensor element are often decreased.